1. Field of the Invention
The present invention relates to a printing apparatus which forms an image by applying a printing agent onto a printing medium from printing means having a plurality of printing elements which are arranged therein. Particularly, the present invention relates to a method and a configuration for controlling a displacement in a printing position of the printing element.
2. Description of the Related Art
In an ink jet printing apparatus, generally, a printing head including a plurality of printing elements integrally arranged is used. Specifically, each of the printing elements includes an ink discharge port and a liquid path for supplying ink to the ink discharge port. Moreover, in order to deal with a color image, many ink jet printing apparatuses include such printing heads for a plurality of colors. The ink jet printing apparatuses are generally classified into a serial type and a line type depending on differences in printing operations thereof. The serial type printing apparatus, which is easily reduced in size, is widely used mainly for personal use in particular.
In such a serial type ink jet printing apparatus, a printing position on a printing medium may include an inclination due to arrangement tolerance of a plurality of ink discharge ports arranged in the printing head, an error in mounting the printing head on the printing apparatus, and the like.
FIG. 1 is a view for explaining the inclination described above. Even if printing of a parallel line pattern in a sub-scanning direction is attempted, a ruled line in a state of being inclination to the sub-scanning direction as shown in FIG. 1 is formed when a printing head having an inclination is used. Here, FIG. 1 shows a case where a displacement for 3 pixels is generated between printing elements positioned at both ends of the printing head at a 1200 dpi (dot/inch) printing resolution. In the serial printing apparatus, an image is formed by intermittently repeating main print scanning for forming an image while moving and scanning the printing head on a printing medium and sub-scanning for transporting the printing medium in a direction which intersects the main print scanning. Thus, even if the ruled line extended in the sub-scanning direction is printed, the line is split into pieces in each print scanning if such an inclination is included. Accordingly, only the ruled line with poor linearity can be obtained. Furthermore, in the case where a color image is formed by including such printing heads for a plurality of colors, other adverse effects, such as occurrence of color shading and worsening of graininess (feeling of roughness when viewing the image) may even be induced.
As described above, in the serial type ink jet printing apparatus, the adverse effect on the image due to the inclination has heretofore been one of significant issues. Moreover, some of various measures for coping with the inclination as described above have already been proposed and implemented.
For example, Japanese Patent Application Laid-open No. 7-309007 (1995) discloses an ink jet printing system including an error correction circuit, which adds an offset to image data to be printed by the respective discharge ports, in order to reduce a printing position error caused by rotation of the printing head. Moreover, Japanese Patent Application Laid-open No. 7-040551 (1995) discloses an ink jet printing apparatus, in which a plurality of discharge port arrays arranged on a printing head are divided into a plurality of blocks, and a sequence of discharge in each block and intervals of discharge are adjusted according to the inclination. Furthermore, No. 11-240143 (1999) discloses a method for printing image data in the following manner. Specifically, in order to correct a displacement, which is caused by the inclination of the printing head, of a printing position in a connection part of each print scanning, an offset is set on the bases of a displacement between a printing position by an uppermost discharge port and a printing position by a lowermost discharge port, and the data is displacement ed by an amount based on the offset for a part of the discharge ports. Furthermore, Japanese Patent Application Laid-open No. 2004-009489 discloses an ink jet printing apparatus having means for changing allocation of data to be printed by the respective discharge ports according to the inclination of the printing head.
FIG. 2 shows an example of executing inclination correction for the printing head having the inclination shown in FIG. 1 by use of the method disclosed in Japanese Patent Application Laid-open No. 11-240143 (1999). In the case of the printing head in FIG. 1, both ends of each of discharge ports are in a state of being away from each other by 3 pixels. Thus, in this example, a plurality of discharge ports (printing elements), which are arranged on the printing head, are divided into 3 blocks 201 to 203. Moreover, the block 202 is set to discharge ink at a timing later (or earlier) than the block 201 by 1 pixel, and the block 203 is set to discharge the ink at a timing later (or earlier) than the block 202 further by 1 pixel. As a result of performing such control, a width d204 of a ruled line after correction is suppressed to be smaller than a width d101 of the ruled line before correction. Thus, a ruled line pattern with better linearity is realized.
Note that, in Japanese Patent Application Laid-open No. 11-240143 (1999), adopted is a method for shifting printing data given to the printing elements in each block by 1 pixel in the main scanning direction in order to discharge the ink while shifting the timing of discharge among the blocks. Meanwhile, according to Japanese Patent Application Laid-open Nos. 7-309007 (1995) and 7-040551 (1995), the following method is disclosed. Specifically, in order to respond to a minute inclination within a range of 1 pixel, a drive signal for discharge is generated while shifting the timing among a plurality of blocks within a time equivalent to 1 pixel region. Both of the methods described above are the same in content that the plurality of printing elements is divided into the plurality of blocks, and that the ink is discharged while shifting the timing among the blocks.
However, even if the methods described in the foregoing patent documents are adopted, sufficient image quality may not be obtained if multi-pass printing is executed by bidirectional print scanning, for example.
Here, the multi-pass printing will be briefly described. In the multi-pass printing, data to be printed in the same image region are divided by use of a plurality of patterns, which are in a complementary relationship, and an image is gradually formed by performing print scanning more than once. Between the respective print scanning, a printing medium is transported by an amount smaller than a printing width of the printing head. Thus, a line leading in the main scanning direction are formed of multiple kinds of printing elements. Printing characteristics of the respective printing elements are, therefore, dispersed in the entire image to smoothen the entire image. Generally, a transportation amount (hereinafter also referred to as an LF width) of the printing medium when the multi-pass printing is carried out is obtained by equally dividing a printing width H of the printing head with respect to the sub-scanning direction by a multi-pass number P. Specifically, the LF width is set to H/2 in printing with 2 passes, and is set to H/6 in printing with 6 passes.
According to the study done by the inventors of the present invention, description will be given for influences of the inclination of the printing head, and of a displacement in a bidirectional printing on an image in bidirectional multi-pass printing, and for problems when conventional inclination correction is carried out. Note that, in the present specification, the displacement in the bidirectional printing is defined as a state where printing positions on the printing medium by forward scanning and backward scanning of the printing head are displacement ed with respect to the main scanning direction. Moreover, a displacement amount of the bidirectional printing is defined as an amount of the displacement.
FIG. 3 shows a state where a ruled line having a width d, which is parallel to a sub-scanning direction; is printed by bidirectional printing with two passes by use of a printing head having no inclination when no displacement in a bidirectional printing exists. In first print scanning, the printing head prints a part of ruled line data in a position shown in FIG. 3 while moving in a forward direction. Subsequently, sub-scanning is performed for a half (that is, H/2) of a printing width. Thereafter, the printing head performs second print scanning while moving in a backward direction. In a region A where the second print scanning and the first print scanning overlap with each other, a combination of dots in a complementary relationship forms a ruled line pattern. As a result of similarly performing third print scanning and fourth print scanning, and further performing a transportation operation between the respective print scanning, a ruled line pattern as shown in FIG. 3 is formed.
Here, a state in an ideal complementary relationship satisfies the following four conditions. First, dots, which form a longitudinal ruled line image, are arranged in a checker array. Secondly, checker array data are formed of binary data. Third, in the case of data corresponding to level 0 among the binary data, no ink is discharged (0 shot). Fourth, in the case of data corresponding to level 1 among the binary data, two shots of ink droplets are discharged. Moreover, the ink droplets are discharged so as to overlap with each other in the same position between the first pass and the second pass.
In this example, since the printing head has no inclination, the ruled line pattern partially printed in each print scanning is parallel to the sub-scanning direction. In addition, since no displacement in the bidirectional printing exists, a region printed in the forward scanning and a region printed in the backward scanning completely overlap with each other. Thus, an image completed by bidirectional multi-pass printing with two passes is set to be a ruled line, which is parallel to the sub-scanning direction, which has the width d, and which is excellent in linearity.
FIG. 4 is an enlarged schematic view for explaining states of dots printed in regions B and C shown in FIG. 3. As is clear from FIG. 4, the complementary relationship is maintained between dots printed in the forward scanning and dots printed in the backward scanning. Accordingly, an even image excellent in dispersibility of dots is formed within a printing region having the width d.
FIG. 5 shows a pattern similar to that shown in FIG. 3, which is formed by the bidirectional printing with two passes by use of the printing head having no inclination when a displacement in the bidirectional printing exists. As in the case of FIG. 3, since the printing head has no inclination, the ruled line pattern partially printed in each print scanning is parallel to the sub-scanning direction. However, since a displacement in the bidirectional printing exists, a region printed in the forward scanning and a region printed in the backward scanning overlap with each other in the state of being shifted from each other in the main scanning direction. Thus, an image completed by bidirectional multi-pass printing with two passes is set to be a ruled line, which is parallel to the sub-scanning direction, but which has a width d′ larger than the width d.
FIG. 6 is an enlarged schematic view for explaining states of dots printed in regions B and C. As is clear from FIG. 6, dots printed in the forward scanning and dots printed in the backward scanning are disposed in positions shifted from each other. Thus, the complementary relationship therebetween is impaired. Within a printing region of such an image, dispersibility of dots is biased, and this is visually recognized as “graininess”. However, since the regions B and C have the same degree of dispersibility of dots, no difference is perceived between the regions.
FIG. 7 shows a pattern similar to that shown in FIG. 3, which is formed by the bidirectional printing-with two passes by use of a printing head having an inclination when no displacement in the bidirectional printing exists. Here, since the printing head has an inclination, the ruled line pattern printed in each print scanning has inclination to the sub-scanning direction. Although no displacement in the bidirectional printing exists, each region is printed by different portions (upper half and lower half) of the printing head respectively in the forward scanning and in the backward scanning. Thus, a region printed in the forward scanning and a region printed in the backward scanning are shifted from each other in the main scanning direction. Thus, an image completed by bidirectional multi-pass printing with two passes is set to be a ruled line having a width d1001 larger than the width d.
FIG. 8 is an enlarged schematic view for explaining states of dots printed in regions B and C. As is clear from FIG. 8, dots printed in the forward scanning and dots printed in the backward scanning are disposed in positions shifted from each other. Thus, the complementary relationship therebetween is impaired. However, since the regions B and C have the same degree of dispersibility of dots, no difference is perceived between the regions.
FIG. 9 shows a pattern similar to that shown in FIG. 3, which is formed by the bidirectional printing with two passes by use of the printing head having an inclination when a displacement in the bidirectional printing exists. Here, since the printing head has an inclination, the ruled line pattern printed in each print scanning has inclination to the sub-scanning direction. In the forward scanning and the backward scanning, each region is printed by different portions (upper half and lower half) of the inclined printing head. Furthermore, the printing is influenced by the displacement in the bidirectional printing. Thus, a displacement amount in the main scanning direction between the forward scanning and the backward scanning varies depending on the region. In FIG. 9, for example, in a region B, there is no displacement between a printing position in the forward scanning and a printing position in the backward scanning. On the other hand, in a region C, there is a large displacement therebetween.
FIG. 10 is an enlarged schematic view for explaining states of dots printed in the regions B and C. As is clear from FIG. 10, in the region B, a complementary relationship is maintained between dots printed in the forward scanning and dots printed in the backward scanning. On the other hand, in the region C, dots printed in the forward scanning and dots printed in the backward scanning are disposed in positions, which are displaced from each other. Thus, in the region C, the complementary relationship is impaired, and “graininess” is perceived more easily than in the region B. In a case where the print scanning is continued further, two regions each having different degrees of “graininess”, such as the regions B and C, are alternately disposed in the sub-scanning direction. This state is recognized as unevenness having a cycle in the sub-scanning direction. As described above, cyclic unevenness caused by a combination of the inclination of the printing head and the displacement in the bidirectional printing will be hereinafter referred to as “band unevenness” in the present specification. Generally, in the bidirectional multi-pass printing, such “band unevenness” is more visible than the “graininess” described above, and is a significant factor which impairs image quality.
Next, description will be given for a printing state when the inclination correction described in the section of the related art is carried out for the printing head having an inclination as shown in FIGS. 7 and 9.
FIG. 11 shows a pattern similar to that shown in FIG. 3, which is formed by bidirectional printing with two passes while correcting an inclination by use of the printing head having an inclination when no displacement in the bidirectional printing exists.
FIG. 12 is a view showing block divisions for correcting the inclination of the printing head. Here, a plurality of printing elements are divided into two blocks of a block 1501 and a block 1502, and timings at which the both blocks are driven are shifted from each other in a direction of inclination correction. With reference to FIG. 11 again, since the inclination correction performed, a width d1401 of the ruled line is set smaller than the width d1001 (FIG. 7) in the state where no inclination correction is performed. Moreover, since no displacement in the bidirectional printing exists, each region is not displaced from each other between the forward scanning and the backward scanning. Thus, “graininess” is maintained in a desirable state in the respective regions.
The above description has been given for the printing state at the time when the bidirectional printing with two passes is executed while performing 2-division inclination correction. Furthermore, in order to examine more cases, a case of bidirectional printing with six passes will be described below.
FIG. 13 shows a pattern similar to that shown in FIG. 3, which is formed by the bidirectional printing with six passes by use of a printing head having an inclination when no displacement in a bidirectional printing exists. In the case of the bidirectional printing with six passes, an image is formed by performing 6 times of print scanning in total, including 3 times of the forward scanning and 3 times of the backward scanning, in each region. Here, since the printing head has an inclination, the ruled line pattern printed in each print scanning inclines to the sub-scanning direction. Although no displacement in the bidirectional printing exists, the forward scanning and the backward scanning for each region are performed each by use of different portions of the inclined printing head. Thus, the region printed by forward scanning and the region printed by backward scanning are displaced from each other in the main scanning direction. Accordingly, an image completed by bidirectional multi-pass printing with six passes is set to be a ruled line having a width d1603 larger than the width d.
Since dots printed in the forward scanning and dots printed in the backward scanning are disposed in positions displaced from each other, the complementary relationship therebetween is impaired. However, the degree of displacement amounts (for example, d1601 and d1602) between printing positions in the forward scanning and printing positions in the backward scanning is the same among a plurality of regions including regions D and E. Thus, the degree of dispersibility of dots is set the same among the respective regions, and no “band unevenness” is perceived.
FIG. 14 shows a pattern similar to that shown in FIG. 3, which is formed by the bidirectional printing with six passes by use of the printing head having an inclination when a displacement in the bidirectional printing exists. As in the case of FIG. 13, since the printing head has an inclination, the ruled line pattern printed in each print scanning is inclined to the sub-scanning direction. Since a plurality of print scanning for each region are performed each by use of different portions of the inclined printing head, the region printed by six scanning are displaced from each other in the main scanning direction. Furthermore, since the displacement in the bidirectional printing exists, displacement widths in the main scanning direction among the plurality of print scannings vary depending on the region. In FIG. 14, for example, a displacement width d1701 in a region D is smaller than a displacement width d1702 in a region E. The fact, as described above, that the displacement width between the print scannings differs in each region leads to a difference in a complementary state of dots among the regions. As a result, occurrence of “band unevenness” is induced.
Next, description will be given for a division method for correcting an inclination of the printing head. FIGS. 15A to 15C are views, which show a plurality of block division methods for correcting the inclination of the printing head, and which show inclination correction states in the respective division methods. FIG. 15A shows a driving state when inclination correction is not executed (top) and a printing result thereof (bottom). When the inclination correction is not executed, all the printing elements of the printing head are driven approximately at the same time, and the printing result is in a state where the inclination of the printing head remains. A ruled line width (hereinafter referred to as an inclination width) in this event is set to be d1806.
FIG. 15B shows a state where the printing elements arranged in the printing head are equally divided into two of upper and lower blocks, and where the respective blocks are driven by shifting timings in a direction for correcting the inclination of the printing head (top), and a printing result thereof (bottom). Since 2-division inclination correction is performed, an inclination width d1807 after correction is set smaller than d1806. Thus, a ruled line having better linearity can be obtained.
FIG. 15C shows a state where the printing elements arranged in the printing head are equally divided into three blocks, and where the respective blocks are driven by shifting timings in the direction for correcting the inclination of the printing head (top), and a printing result thereof (bottom). Since 3-division inclination correction is performed, an inclination width d1808 after correction is set smaller than the inclination width d1807 after the 2-division inclination correction. Thus, a ruled line having better linearity can be obtained.
FIG. 16 shows a ruled line pattern formed by bidirectional printing with six passes while performing 2-division inclination correction by use of the printing head having an inclination when no displacement in a bidirectional printing exists. Since the inclination correction is performed, a width d1901 of an image is set to be further smaller than the width d1603 obtained when no inclination correction is performed. Although no displacement in the bidirectional printing exists, each region includes some displacements between the forward scanning and the backward scanning, due to the configuration in which the 2-division inclination correction and multi-pass printing with six passes are performed. However, since such a displacement amount is not changed for each region, no “band unevenness” occurs.
FIG. 17 shows a ruled line pattern formed by the bidirectional printing with six passes while performing 3-division inclination correction by use of the printing head having an inclination when no displacement in the bidirectional printing exists. Since the inclination correction is performed, a width d2001 of an image is set to be further smaller than that in the case of the 2-division inclination correction shown in FIG. 16. Although no displacement in the bidirectional printing exists , each region includes some displacement s between the forward scanning and the backward scanning, due to the configuration in which the 3-division inclination correction and the multi-pass printing with six passes are performed. However, as in the case of FIG. 16, since such a displacement amount is not changed for each region, no “band unevenness” occurs.
According to the above description with reference to FIGS. 15A to 17, the larger the division number for inclination correction is, the more the displacement width of the printing position in the main scanning direction can be suppressed. Thus, it can be determined that an image can be formed in a desirable state. However, the keen examination by the inventors of the present invention has confirmed that, if the displacement in the bidirectional printing exists, a desirable image is not necessarily formed by correction using a larger division number.
FIGS. 18A and 18B are views showing how the bidirectional printing with six passes is carried out while performing 2-division inclination correction in a state where the printing head having an inclination is used, and where a displacement in the bidirectional printing exists. FIG. 18A is a view showing printing positions of two blocks when the respective blocks are driven while timings therebetween are shifted. In the bidirectional printing with six passes, main scanning and sub-scanning are alternately repeated. Specifically, in the main scanning, ink is discharged while the printing head in the printing state as described above is bidirectionally moved. Moreover, in the sub-scanning, a printing medium is transported by an amount corresponding to an LF width (=H/6) shown in FIG. 18A. Thus, for example, when a printing region D of the printing medium is in focus, in the region, the forward scanning is performed by use of portions 2301 and 2303 of a first block and a portion 2305 of a second block, and the backward scanning is performed by use of a portion 2302 of the first block and portions 2304 and 2306 of the second block. Both of a sum of the portions printed by the forward scanning and a sum of the portions printed by the backward scanning cover the entire inclination width d1807 (see FIG. 15B) after the inclination correction.
Meanwhile, in a printing region E adjacent to the region D, an image is formed in a state where the forward scanning and the backward scanning are reversed from those in the region D. Specifically, the forward scanning is performed by use of the portion 2302 of the first block and the portions 2304 and 2306 of the second block, and the backward scanning is performed by use of the portions 2301 and 2303 of the first block and the portion 2305 of the second block. Thus, as in the case of the region D, both of the sum of the portions printed by the forward scanning and the sum of the portions printed by the backward scanning cover the entire inclination width d1807 after the inclination correction.
FIG. 18B shows a ruled line pattern printed in the printing state as described above. Since the displacement in the bidirectional printing exists, each region includes displacement s between the forward scanning and the backward scanning. Thus, printing widths d2101 and d2102 are set further larger than the inclination width d1807 after the inclination correction. However, since such a displacement amount is not changed for each region, no “band unevenness” occurs.
FIGS. 19A and 19B are views showing how the bidirectional printing with six passes is carried out while performing 3-division inclination correction in a state where the printing head having an inclination equivalent to that shown in FIGS. 18A and 18B is used, and where a displacement in the bidirectional printing exists. FIG. 19A is a view showing printing positions of three blocks when the respective blocks are driven while timings therebetween are shifted. In the bidirectional printing with six passes, main scanning and sub-scanning are alternately repeated. Specifically, in the main scanning, ink is discharged while the printing head in the printing state as described above is bidirectionally moved. Moreover, in the sub-scanning, a printing medium is transported by an amount corresponding to an LF width (=H/6) shown in FIG. 19A. Thus, for example, when a printing region D of the printing medium is in focus, in the region, the forward scanning is performed by use of a portion 2401 of a first block, a portion 2403 of a second block and a portion 2405 of a third block, and the backward scanning is performed by use of a portion 2402 of the first block, a portion 2404 of the second block and a portion 2406 of the third block. In the case of this example, the sum of the portions printed by the forward scanning and the sum of the portions printed by the backward scanning each occupy a different half of a region of the inclination width d1808 of the printing head after the inclination correction.
Meanwhile, in a printing region E adjacent to the region D, an image is formed in a state where the forward scanning and the backward scanning are reversed. Specifically, the forward scanning is performed by use of the portion 2402 of the first block, the portion 2404 of the second block and the portion 2406 of the third block, and the backward scanning is performed by use of the portion 2401 of the first block, the portion 2403 of the second block and the portion 2405 of the third block. Thus, the sum of the portions printed by the forward scanning and the sum of the portions printed by the backward scanning each occupy a different half of the region of the inclination width d1808 of the printing head after the inclination correction. However, a region occupied by the sum of the forward scanning and a region occupied by the sum of the backward scanning are in a positional relationship reversed from that in the region D. If the displacement in the bidirectional printing exists in such a state, the printing widths in the regions D and the region E are changed in opposite directions each of increase and reduction. Specifically, when the printing width in the region D is increased, the printing width in the region E is reduced, and when the printing width in the region D is reduced, the printing width in the region E is increased.
FIG. 19B shows a ruled line pattern printed in the printing state as described above. Since the displacement in the bidirectional printing exists, each region includes displacement s between the forward scanning and the backward scanning. Thus, printing widths d2201 and d2202 in the respective regions are set to be different from the width d1808 after the inclination correction. In this event, the printing width d2201 in the region D is larger than the width d1808 after the inclination correction. On the other hand, the printing width d2202 in the region E is smaller than the width d1808 after the inclination correction. As described above, a change in a displacement amount (that is the printing width) between an image formed by the forward scanning and an image formed by the backward scanning for each region causes a difference in a dot dispersion state between the respective regions. Furthermore, since regions having different dispersion states, such as the regions D and E, are alternately disposed in the sub-scanning direction, “band unevenness” occurs.
FIG. 41 is a table collectively showing the results described above with-reference to FIGS. 15A to 19B. It is confirmed that the 3-division correction, which corrects the inclination in a more desirable state, may worsen the “band unevenness” rather than the 2-division correction.
As a result of the keen examination, the inventors of the present invention have recognized the phenomenon described above and discovered that it is required to maintain a predetermined relationship between the number of multi-passes and the number of blocks for inclination correction in a case where the inclination correction is performed in the bidirectional multi-pass printing. Specifically, the inventors of the present invention have determined that, in order to prevent occurrence of the “band unevenness”, it is desired to adjust block division for the inclination correction according to the number of multi-passes in the bidirectional printing even if the bidirectional printing is somewhat displaced.
FIGS. 18A and 19A are referred to again to clarify what the aforementioned predetermined relationship is. The reason why the “band unevenness” is avoided in the state of FIG. 18A is because both of the sum of the printing by the forward scanning and the sum of the printing by the backward scanning cover the entire inclination width after the inclination correction. In such a state, even if the sum of the forward scanning and the sum of the backward scanning are reversed, such as the regions D and E, both of the printing regions can maintain a congruent relationship regardless of a displace direction.
As one of conditions for allowing both of the sum of the forward scanning and the sum of the backward scanning to cover the entire inclination width after the inclination correction, there is a condition that “each printing region corresponds to more than one divided blocks”. The size of the inclination width after the inclination correction corresponds to an inclination width of each block. Thus, if each printing region includes more than one divided block, printing over the entire inclination width is completed by performing the forward scanning or the backward scanning once.
Moreover, as another condition, there is a condition that “each of divided blocks corresponds to odd printing regions (one printing region is the printing head width H/the number of multi-passes)”. In the case of FIG. 18A, each of the first and second blocks, which are obtained by division into two blocks, corresponds to three printing regions. As described above, if the number of printing regions corresponding to each block is an odd number, a positional relationship between a forward scanning region and a backward scanning region, which are alternately disposed, is reversed for each block. Thus, both of the sum of the forward scanning and the sum of the backward scanning can cover the entire inclination region after the inclination correction. Meanwhile, in the case of FIG. 19A, each of the blocks, which are obtained by division into three-blocks, corresponds to two printing regions. In such a case, a positional relationship between the forward scanning region and the backward scanning region, which are alternately disposed, is not reversed for each block. Thus, neither the sum of the forward scanning nor the sum of the backward scanning can cover the entire inclination region after the inclination correction.
Specifically, in order to obtain effects of the present invention, it is required to satisfy any one of the first condition “each printing region corresponds to more than one divided blocks” and the second condition “each of divided blocks corresponds to odd printing regions”. A block length after the block division is expressed as a value (H/B) obtained by dividing the printing head width H by the block number B. A width of the printing region is expressed as a value (H/P) obtained by dividing the printing head width H by the multi-pass number P. Thus, the first condition can be expressed as H/P=N×H/B, in other words, B=N×P (N is an integer of not less than 1). Moreover, the second condition can be expressed as H/B=N×H/P, in other words, P=M×B (M is an odd number of not less than 1). In FIG. 23, spots, which satisfy the foregoing conditions, are indicated by circles.
A recent general ink jet printing apparatus often includes means for adjusting a displacement in a bidirectional printing. The printing apparatus is designed to reduce problems with images, such as “graininess” and “ruled line displacement”, as much as possible. However, a slight displacement in the bidirectional printing as described above is a phenomenon likely to occur suddenly due to various factors such as deformation of the printing medium, a variation in a movement speed of the printing head and an ink discharge state of the printing head. Moreover, the inventors of the present invention recognize that the adverse effect of the “band unevenness” described above is an item, which significantly deteriorates image quality, even if the “band unevenness” is a sudden phenomenon. However, in the conventional inclination correction as described in the foregoing patent documents, the division number in the inclination correction is set regardless of the multi-pass number. Thus, it is difficult to avoid the aforementioned “band unevenness”.